1. Field of the Invention
The present invention relates to a drive system for an electric motor vehicle, and more particularly to a support structure associated with drive members connected to drive wheels, which structure is adapted to effectively receive thrust forces applied to the drive members through the drive wheels in the transverse direction of the vehicle.
2. Discussion of the Related Art
For an electric motor vehicle, there is used a drive system of the type which includes (a) a housing mounted on a chassis of the vehicle, (b) an electric motor disposed in this housing and having a hollow output shaft whose axis is parallel to the transverse direction of the vehicle, (c) a first and a second drive member which are supported by the above-indicated housing rotatably about respective axes coaxial with the output shaft of the motor and which are operatively connected to a pair of drive wheels, respectively, (d) an intermediate shaft disposed coaxially with the output shaft of the motor so as to extend through a center bore of the output shaft, and connected at an axial end thereof to the second drive member for rotation therewith, (e) a differential gear device of planetary gear type disposed in the above-indicated housing coaxially and in series with the output shaft of the motor, the differential gear device including four elements consisting of a sun gear, a ring gear, a carrier and a planetary gear, one of which serves as an input element that receives power transmitted from the motor, and the others of which include two output elements through which the power received by the input member are distributed to the first drive member and the intermediate shaft, and (f) a plurality of thrust bearings for positioning the four elements of the differential gear device so as to inhibit movements of these elements in an axial direction of the output shaft of the motor. An example of such drive system for an electric motor vehicle is disclosed in Japanese Patent Application No. 4-160235 filed in the name of the assignee of the present application, which had not been laid open at the time the present invention was made. FIG. 9 of the present application is a cross sectional view of the drive system as disclosed in the above-identified application.
In the electric motor vehicle drive system disclosed in FIG. 9, the power produced by an electric motor 200 is first transmitted to a speed reducing device of planetary gear type 204 through an output shaft 202 of the motor. The power whose torque has been amplified by the speed reducing device 204 at a predetermined speed reduction ratio of the device 204 is then received by a differential gear device 206 of planetary gear type, which has a ring gear 208, a planetary gear 210, a carrier 212 and a sung gear 214. The power received by the ring gear 208 is distributed to the carrier 212 and sun gear 214 through the planetary gear 210, so that a portion of the received power is transmitted from the carrier 212 to a left axle and a left drive wheel (not shown) through a first drive member 216L connected to the carrier 212, while the rest of the power is transmitted from the sun gear 214 to a right axle and a right drive wheel (not shown) through an intermediate shaft 218 and a second drive member 216R. The intermediate shaft 218 is connected at one axis end thereof to the sun gear 214 and extends through the output shaft 202 of the electric motor 200. The other end of the intermediate shaft 218 is connected to the right drive member 216R.
In the drive system of FIG. 9, a thrust force which is applied to the first or second drive member 216L, 216R in the transverse direction and inwardly of the vehicle is received by a housing of the drive system. Such transverse thrust force acts on the first or second drive member 216L, 216R due to an inertial force during turning of the vehicle along a curve or corner, or upon collision of the vehicle with a sidewalk or pavement guard or a guard rail due to slipping of the drive wheels on a road surface with a low coefficient of friction. When a transverse thrust force is applied to the first or left drive member 216L, the thrust force is transmitted to a housing 238 through a thrust bearing 220, the above-indicated sun gear 214, a thrust bearing 222, a second-stage carrier 224 of the speed reducing device 204, a thrust bearing 226, a second-stage sun gear 228 and a first-stage carrier 230 of the device 204, a thrust bearing 232, a first-stage sun gear 234 of the device 204 and a thrust bearing 236. When a transverse thrust force is applied to the second or right drive member 216R, the thrust force is transmitted to a housing 244 through a flange 240 formed to extend radially outwardly of the drive member 216R, and a thrust bearing 242.
The thrust bearing 242 disposed adjacent to the radial outward flange 240 of the second drive member 216R has a comparatively large diameter, and causes a considerably large amount of power loss due to friction when a transverse thrust force input to the second drive member 216R is transmitted to the housing 244 through the flange 240 and thrust bearing 242. Thus, the power transmission efficiency of the drive system is deteriorated. The drive system also suffers from a drawback that the first drive member 216L cannot be removed from the differential gear device 206 for a repairing purpose with the vehicle in a loaded condition, since a radial outward flange 246 formed on the first drive member 216L to support the carrier pin is located within the housing 238 and extends in the radially outward direction so that the housing 238 disturbs the removal of the first drive member 216L.
The drive system of an electric motor vehicle may use a differential gear device of bevel gear type which has been widely used for a motor vehicle with a gasoline engine or other internal combustion engine. This type of differential gear device, which is shown in FIG. 10, is adapted such that a transverse thrust force applied to a left or right drive member 250L, 250R is transmitted to and received by a housing 260 thorough a differential pinion shaft 252, a differential casing 254 and a left or right tapered roller bearing 256, 258. Thus, the drive members 250L, 250R do not have a flange for transmitting the transverse thrust force. If such differential gear device is used in a drive system for an electric motor vehicle, it is necessary to replace the drive members 250L, 250R with exclusive drive members having flanges and provide respective thrust bearings to transmit the received thrust forces. Accordingly, the cost of manufacture of the drive system is unfavorably increased with an increased number of components.